The diffusion redistribution of hydrogen and oxygen in porous silicon films

Abstract

A porous silicon (PS) layer prepared by electrochemical anodization of a single crystal silicon surface has been known for many years. Crystalline structure of the PS is network of silicon is nanometer-sized regions, surrounded by void space, with very large surface to volume ratios [1]. The PS surfaces are covered by silicon hydrides and silicon oxides. Observation of visible photoluminescence from PS has generated widespread interest because of the potential application of PS in Si-based optoelectronic technology. The luminescence, optical and other properties of PS are determined by incorporation of hydrogen and oxygen on surfaces and the formation of Si-H, Si-H2 and Si-O bonds. Therefore stability of luminescence and optical characteristics of PS depend on processes of concentration redistribution of hydrogen and oxygen atoms under treatments. This, in turn, is limited by oxygen and hydrogen diffusion along inner PS surfaces with simultaneous penetration into silicon. The formation mechanism of PS and the luminescence mechanism have been actively discussed, whereas the composition changes of PS caused by hydrogen and oxygen diffusion, and by appropriate changes in the number of Si-H, Si-H2 and Si-O bonds due to thermal treatments are rarely considered. From FTIR measurements, a reduction of photoluminescence intensity of PS with simultaneous decrease of intensity of Si-H2 peaks as results of annealing at 25–400 ◦C was observed by Tsai et al. [2]. Hydrogenand oxygenrelated bonds changes during PS oxidation for the range 20–500 ◦C at atmospheric pressure has also been reported [3, 4]. The aim of the present work is to determine the diffusion parameters of hydrogen and oxygen atoms in free-standing PS films from analysis of changes intensity of Si-H and Si-O absorption peaks as a function of temperature and duration of annealing in air. The PS films were prepared on n-type Si (100) substrates of resistivity ρ= 0,09A cm by anodic etching in a solution HF : H2O= 1 : 3 at a current density of 15 mA/cm2 under the white light illumination. The PS films were then detached from silicon substrate by electropolishing under former solution with a current density of 0.8–1.0 A/cm2. The free-standing PS films and PS film on Si substrates were characterized by porosity, photoluminescence and FTIR measurements at room temperature. Porosity of films was determined by gravimetry. Freestanding PS films of thickness 10–20μm and porosities 70–80% were obtained. FTIR spectra of free-standing PS films were measured at room temperature by using a Mattson 1000 spectrometer. The photoluminescence spectra were excited by the 325 nm line of He-Cd laser under a power output of 20 mW. This investigation focuses on the analysis of changes of absorption coefficients of Si-H, Si-H2 (doublet at 2100 cm−1) and Si-O (1100 cm−1) peaks as a result of isothermal annealing at temperatures in the range 65–185 ◦C at atmospheric pressure. Isothermal annealings of PS films were carried out at 65, 80, 110, 150 and 185 ◦C. Time-dependence FTIR and photoluminescence spectra were measured after 5-min cycles of isothermal annealings. The distribution of photoluminescence intensity along thickness of PS films (on silicon substrates) has been analyzed by sequential removal of thin layers (about 1μm) from PS film and the photoluminescence intensity measurements. The absorption coefficient (α) is deduced from transmission spectra by solving α in the equation